An information recording medium using high energy density beam such as a laser beam for recording and reading has been developed in recent years and is now put into practical use. The information recording medium is called optical disc and can be used as a video disc, audio disc and disc memory for large-capacity static image files and large-capacity computers.
The optical disc basically comprises a recording layer provided on a disc-shaped transparent substrate of a plastic material or glass. An undercoat layer or intermediate layer of a polymer material can be provided on the surface (on which the recording layer is provided) of the substrate for improving smoothness of the surface, adhesion between the substrate and the recording layer and the light sensitivity of the optical disc.
Writing of information on the optical disc or reading of information from the recording disc is conducted in an information recording or reproducing apparatus by irradiating the recording layer of the disc with a laser beam while rotating the disc on its plane. For the rotation of the optical disc, the disc is mounted onto a rotatable spindle of the apparatus. For facilitating the fitting of an optical disc to the spindle, a separately prepared hub can be provided to the disc. Further, it has been proposed that the hub is made magnetizable so as to more firmly fix the optical disc onto a rotating element of the recording or reproducing appartus. The magnetizable hub is provided with magnetizable (magnetic) substance such as a magnetizable metal plate. Such hub is firmly fitted onto a rotating magnet element of a recording or reproducing apparatus, and therefore a recording medium having such magnetizable hub is firmly mounted onto the rotating element.
The hub has a center hole for insertion of a spindle of the rotating element, and is generally comprises a boss (cylinderical part) and a flange. The hub is joined to an information recording disc in such manner that its boss is inserted into a circular hole of an information recording disc which is formed at the center thereof. The hub is arranged within the information recording disc in such a manner that the hub is made concentric to the center of the disc.
Heretofore, joining of the hub to the information recording disc is conducted by the steps of fixing the disc to a rotatable X-Y table, determining the center of the groove formed on the information recording disc by observing the groove through a microscope while making the X-Y table to rotate, adjusting the X-Y table in such a manner that the center of the groove, which is the center of the information recording disc, coincides with the center of a rotation shaft of the X-Y table, and finally setting the hub to the center of the rotating shaft of the information recording disc to join the hub to the information recording disc. Adhesion or ultrasonic welding or the like can be used for the joining.
Thus, an information recording medium provided with a magnetizable hub has been required to adjust the joining location of the hub as described above. There have been known two different types of hubs; one has a boss having smaller diameter (smaller outer diameter) than that of the circular hole of the information recording disc so as to enable the desired adjustment of the location on joining, and the other has no boss. Therefore, there likely arises eccentricity (locational deviation of the center of the information recording disc and the center of the hole for insertion of a spindle) when joining the hub and the information recording disc together.
In regard to an optical disc having a diameter of 130 mm (5.25 inches), there has been a standard defined by ISO (International Organization for Standardization) with respect to the form and the characteristics of a optical disc employing a magnetizable hub. According to the ISO Standard, it is required that the above-described ecentricity be kept within 25 .mu.m.
In the conventional process for producing an information recording disc, the step for determining the center of the disc and then joining the hub to the center is done manually for the most part, and the accuracy of the joining depends greatly on the worker's skills. Moreover, it takes not a short period such of time as several minutes to assemble per disc, which gives poor productivity. For this reason, several attempts have been made to conduct a series of the centering processes (which is made to determine the center of the information recording disc) automatically by machine. However, the centering processes using a known machine requires a complicated treatment system as well as high manufacturing cost. Moreover, the use of a known machine for the centering does not give satisfactory increase of productivity.